1. Field of the Invention
The present invention relates to a process for producing microcapsules by encapsulating hydrophobic materials (liquids or solid powders) in a polar solvent. More particularly, the present invention relates to a process for producing microcapsules using polyfunctional amines in combination with reactive organic compounds which are capable of reacting with the polyfunctional amines to form substances insoluble in the polar solvent.
2. Description of the Prior Art
Various processes for encapsulation of a hydrophobic material in a polar solvent are known.
The apparent state and properties of a material can be changed by encapsulation. That is, the material can be protected in the microencapsulated state and the ability of the material (the capsule contents) to be released can be controlled. Moreover, the capsule contents can be released all at once when desired.
The functions of the microcapsule are as follows:
1. A liquid can be seemingly changed to a solid.
2. The weight and volume of a material can be changed.
3. The release of the contents can be controlled.
4. Two or more reactive materials can be isolated and stored together for a long period. The contents can be protected from the atmosphere or stored for a desired period.
5. The contents can be prevented from coloring, the taste of the contents can be masked and toxic materials can be protected.
6. Microcapsules are fine particles.
In view of these functions, applications of microcapsules to recording materials, medicines, perfumes, agricultural medicines, chemicals, adhesives, liquid crystal paints, foods, detergents, dyes, solvents, catalysts, enzymes, anti-corrosive agents and the like have been investigated. Thus, nowadays, pressure sensitive copying papers, aspirin capsules, perfume capsules, menthol capsules, adhesive capsules, capsules containing an anti-corrosive agent for rivets, liquid crystal capsules, insecticide capsules and the like have been commercially produced, and various encapsulation methods are skillfully utilized depending upon the capsule contents and the use of the capsules.
A number of encapsulation methods can be classified into chemical, physical-chemical, and mechanical-physical methods, or a combination of these methods.
These encapsulation methods are explained in greater detail hereinbelow.
Encapsulation methods employing a chemical method include an interfacial polymerization method and an in situ polymerization method. The interfacial polymerization method utilizes a polymerization reaction per se, and is described in Polymer Science, 60, 299 (1959). In this case, an interfacial polymerization is carried out combining hydrophobic monomers (initial reaction products are also used) and hydrophilic monomers (initial reaction products are also used). A hydrophobic monomer is incorporated into an organic solvent which does not have any affinity for water, which is dispersed in a water phase. When a water-soluble or water-dispersible monomer is added to the water phase, polymerization proceeds at the water and oil interface, thus resulting in the formation of a polymer film. Compounds suitable in forming the film are polyfunctional materials capable of undergoing polycondensation or polyaddition reactions, and thus a polyamide, polyester, polyurethane, or polyurea wall is formed.
Encapsulation methods employing the above principle are described in Japanese Pat. Publication Nos. 19574/1963, 446/1967, 771/1967, 2882/1967, 2883/1967, 8693/1967, 8923/1967, 9654/1967, and 11344/1967; British Pat. Nos. 950,443, 1,046,409, and 1,091,141; and U.S. Pat. Nos. 3,577,515 and 3,492,380.
The capsule wall as prepared by the above-described method is a typical semipermeable membrane because the speed at which the monomer reacts is reduced with formation of the capsule wall and finally stops, thereby resulting in the formation of a thin film and the presence of unreacted monomer.
Moreover, a specific selectivity between the capsule contents and the capsule wall forming material is required, and the capsule contents are affected by the capsule wall forming material. Thus, the kind of material which can be encapuslated is limited.
In the in situ polymerization method, the capsule wall forming material is fed only from one side of the interface, i.e., either from the inside or from outside the core material droplet, and thus the polymerization proceeds necessarily at the surface of the core material droplet. Almost all known polymerization methods can be employed and a variety of types of capsule walls can be formed.
The former method in which an oily monomer is present in the core material, is described in Japanese Pat. Publication No. 9168/1961, British Pat. No. 1,237,498, French Pat. Nos. 2,060,818, and 2,090,862. The latter method in which the capsule wall forming material is fed from the side of the medium and a polymer membrane is formed on the surface of the core material droplet, is described in British Pat. No. 989,264 and Japanese Pat. Publication Nos. 14327/1962, 12380/1962, 7313/1971, 29483/1970 and 30282/1971.
The capsule wall as prepared by these methods, with a few exceptions, also suffers from a wall formation which is still insufficient and the wall is still too porous.
Furthermore, problems often arise in that a specific selectivity between the capsule contents and the capsule wall forming material is required, the capsule contents are adversely affected by the capsule wall forming material, and some difficulty is encountered in encapsulation.
The encapsulation method employing a physical-chemical method includes a phase-separation from an aqueous solution, a drying in a liquid, and the like. The phase-separation from an aqueous solution comprises separating a phase rich in a polymer from an aqueous solution of the polymer, and it has been most widely commercialized and the greatest number of attempts toward utilization of this method have been made.
As methods, there are a complex coacervation method and a simple coacervation method, e.g., in which gelatin is used as a hydrophilic polymer.
The complex coacervation method is described in U.S. Pat. Nos. 2,800,457, 3,116,206, 3,265,630, 3,190,836, and 3,041,289. The methods of curing the above formed capsule wall are described in Japanese Pat. Publication Nos. 3878/1962, 3876/1962, 3877/1962, 12376/1962 and 24782/1964, and U.S. Pat. No. 3,401,123. As an agent for curing the capsule wall, formaldehyde, glyoxal, glutaraldehyde, and the like are often used.
Since the capsule wall prepared by the above methods is formed from a water soluble polymer, the capsule wall inherently has low resistance to water or humidity and thus swells and the capsule contents exude. Moreover, since the capsule wall formed is porous, low molecular substances tend to pass therethrough. The capsule contents are easily extracted with alcohols, esters and ketones.
Methods employing the simple coacervation method are described in U.S. Pat. No. 2,800,458, French Pat. No. 1,304,891, Japanese Pat. Publication Nos. 7727/1962, 7731/1962 and 9681/1962.
The capsule wall as prepared by the above simple coacervation methods has similar properties to those as prepared by the complex coacervation method.
In the method of drying in liquid, a solution of a capsule wall forming material containing a core material is dispersed in an encapsulation medium of water or oil and a solvent is evaporated to form a hard capsule wall.
This method is described in Japanese Pat. Publication Nos. 13703/1967, 28744/1964 and 28745/1964.
The capsule wall as formed by this method is generally a thin semipermeable membrane. Thus, where the core material is of low molecular weight, disadvantageously the core material passes through the capsule wall.
In addition to the above described encapsulation methods, a phase-separation from an organic phase method (see, for example, Japanese Pat. Publication No. 12379/1962 and U.S. Pat. No. 3,173,878) and a liquid drying method (see, for example, Japanese Pat. Publication Nos. 28744/1964, 28755/1964 and 13703/1967) are known. However, these methods do not provide walls of a sufficient thickness and the formed capsule wall is not sufficiently thin.